Based on processing different feedstock in the steam reformer, it is known that carbon conversion improves with reformer or first stage gasifier operating temperature. Unfortunately, with an increase in reformer operating temperature, for a specified pulsed heater capacity, the throughput decreases. This decrease in throughput is a result of the decrease in temperature difference between the heater tubes and the fluidized bed that occurs with an increase in reformer operating temperature, at a constant pulse heater firing rate. Consequently, the heat dumping rate decreases while the heat required per unit weight of feed increases due to the higher sensible heat requirement. The net result is a decrease in feedstock throughput at higher reformer operating temperatures.
The demands on the pulsed heater metallurgy also become severe with an increase in operating temperature and the feedstock contamination level (type and quantity of contaminants such as S, Cl, metals, etc.). The reformer temperature is usually not increased to its maximum potential but instead is set at a rate lower than the maximum, depending on the feedstock, pulsed heater metallurgy and the process application. This compromise in reformer temperature results in carbon conversion that may not approach the 98%-99% conversion target, if the feedstock fixed carbon content is relatively high and/or the char resulting from devolatilization is relatively less reactive. To ensure high carbon conversion in these cases, an option is to include a char converter or carbon trim cell downstream of the steam reformer secondary cyclone. This converter provides additional residence time and a reactive environment for the char fines caught by the secondary cyclone. This converter operates as a trim fluidized bed with an oxygen containing gas (air or enriched air or oxygen) as the fluidization medium and in a substoichiometric mode to gasify the carbon to primarily CO. The fluidized bed may be of the circulating bed type or turbulent bed type or bubbling bed type. The fluidized bed operating temperature selected may range from 650° C. (or 1,200 F.) to 1,200° C. (or 2,192° F.) depending upon the char reaction and ash softening/agglomeration characteristics. Steam is added, as necessary, to facilitate material transfer, regulate temperature in the converter and avoid ash fusion. The products from the converter pass through third stage cyclone for solids retention and recycle and fourth stage cyclone to drop out the fly ash; the CO-rich gas then is blended with the second cyclone off-gas or syngas (comprising primarily H2 and CO) and routed to a heat recovery steam generator (HRSG). The relatively high CO content of the product gas from the char converter helps to compensate for the high H2 content of the steam reformer exit gas and produce a blend that has a H2 to CO ratio of ˜2 on a molar basis which is ideal for biofuel production.